Very Low Frequency (VLF, 3-30 kHz) waves have long been used for global submarine communications and navigation. Although lightning is a frequent and intense source of VLF waves, anthropogenic generation of broadband VLF waves remains difficult because of the extremely long wavelengths (10-100 km). Development of global VLF-based navigation systems, as well as underground remote sensing with VLF waves that penetrate into the ground, is hampered by the inability to generate broadband VLF waves.